Double-implant high performance varactor and method for manufacturing same

Abstract

A varactor designed to enable voltage controlled oscillator (VCO) integration in wireless systems is the base-emitter junction of a specially optimized NPN device formed with a double base implant. A first, shallow implant optimizes capacitance, leakage current, and tuning range. A second, deeper base implant is used to improve the quality factor of the device by reducing the base resistance. The varactor includes a third terminal (collector), which isolates the emitter-base junction from the substrate, providing flexibility in circuit applications. A method for fabricating a high performance varactor having the above-described structure is also provided.

1. A method of forming a varactor device on a semiconductor substrate, comprising the steps of:
 forming a buried layer in said semiconductor substrate, said semiconductor substrate having a first conductivity type and said buried layer having a second conductivity type; 
 providing an epitaxial layer situated in said semiconductor substrate, said epitaxial layer having said first conductivity type, wherein said epitaxial layer is situated over said buried layer; 
 providing an isolation structure on said semiconductor substrate, said isolation structure defining an implant region, said implant region being situated over said epitaxial layer; 
 selecting a first peak dopant concentration and a first implant energy such that at least one of capacitance, leakage current, and tuning range of the varactor device is optimized; 
 forming a first implant in said epitaxial layer using said first implant energy, said first implant having said first peak dopant concentration and said first conductivity type, wherein said first implant extends into said epitaxial layer a first distance, and wherein said first implant is situated over said buried layer; 
 forming a second implant in said epitaxial layer using a second implant energy, said second implant having a second peak dopant concentration and said first conductivity type, wherein said second implant extends into said epitaxial layer a second distance, wherein said second distance is greater than said first distance, wherein said second implant is situated over said buried layer, 
 and wherein said second peak dopant concentration has a depth that is greater than a depth of said first peak dopant concentration. 
 
   
   
     2. A method in accordance with  claim 1 , further comprising the step of annealing the device following the steps of forming said first implant and said second implant. 
   
   
     3. A method in accordance with  claim 1 , wherein said selecting step comprises determining an as-implanted dopant concentration profile for said first implant. 
   
   
     4. A method in accordance with  claim 3 , wherein said step of determining an as-implanted dopant concentration profile is performed using secondary ion mass spectroscopy. 
   
   
     5. A method in accordance with  claim 1 , further comprising the step of selecting said second peak dopant concentration and said second implant energy such that the base resistance of the varactor device is minimized. 
   
   
     6. A method in accordance with  claim 5 , wherein said selecting step comprises determining an as-implanted dopant concentration profile for said second implant. 
   
   
     7. A method in accordance with  claim 6 , wherein said step of determining an as-implanted dopant concentration profile is performed using secondary ion mass spectroscopy. 
   
   
     8. A method in accordance with  claim 1 , further comprising the step of forming a contact layer of said second conductivity type overlying said first implant. 
   
   
     9. A method in accordance with  claim 1  wherein said step of providing an isolation structure comprises providing an isolation structure including a CMOS well. 
   
   
     10. A method of forming a varactor device on a semiconductor substrate, comprising the steps of:
 forming a buried layer in said semiconductor substrate, said semiconductor substrate having a first conductivity type and said buried layer having a second conductivity type; 
 providing an epitaxial layer situated in said semiconductor substrate, said epitaxial layer having said first conductivity type, wherein said epitaxial layer is situated over said buried layer; 
 providing an isolation structure on said semiconductor substrate, said isolation structure defining an implant region, said implant region being situated over said epitaxial layer; 
 forming a first implant in said epitaxial layer using a first implant energy, said first implant having a first peak dopant concentration and said first conductivity type, wherein said first implant extends into said epitaxial layer a first distance, and wherein said first implant is situated over said buried layer; 
 forming a second implant in said implant region of said isolation structure using a second implant energy, said second implant having a second peak dopant concentration and said first conductivity type, wherein said second implant extends into said epitaxial layer a second distance, wherein said second implant is situated over said buried layer, 
 wherein said second distance is greater than said first distance, wherein said first peak dopant concentration and said first implant energy are selected such that at least one of capacitance, leakage current, and tuning range of the varactor device are optimized, and wherein said second peak dopant concentration and said second implant energy are selected with relation to said first peak dopant concentration and said first implant energy such that the base resistance of the varactor device is minimized, 
 and wherein said second peak dopant concentration has a depth that is greater than a depth of said first peak dopant concentration. 
 
   
   
     11. A method in accordance with  claim 10  further comprising the step of forming a contact layer of said second conductivity type overlying said first implant. 
   
   
     12. A method in accordance with  claim 10  wherein said step of providing an isolation structure comprises providing an isolation structure including a CMOS well. 
   
   
     13. A method of forming a varactor device on a semiconductor substrate, comprising steps of:
 forming a buried layer in said semiconductor substrate, said semiconductor substrate having a first conductivity type and said buried layer having a second conductivity type; 
 providing an epitaxial layer situated in said semiconductor substrate, said epitaxial layer having said first conductivity type, wherein said epitaxial layer is situated over said buried layer; 
 selecting a first peak dopant concentration and a first implant energy such that at least one of capacitance, leakage current, and tuning range of the varactor device is optimized; 
 forming a first implant in said epitaxial layer using said first implant energy, said first implant having said first peak dopant concentration and said first conductivity type, wherein said first implant extends into said epitaxial layer a first distance, and wherein said first implant is situated over said buried layer; 
 forming a second implant in said epitaxial layer using a second implant energy, said second implant having a second peak dopant concentration and said first conductivity type, wherein said second implant extends into said epitaxial layer a second distance, wherein said second distance is greater than said first distance, wherein said second implant is situated over said buried layer, 
 and wherein said second peak dopant concentration has a depth that is greater than a depth of said first peak dopant concentration. 
 
   
   
     14. A method in accordance with  claim 13 , further comprising the step of forming a contact layer of said second conductivity type overlying said first implant. 
   
   
     15. A method in accordance with  claim 13 , further comprising the step of annealing the device following the steps of forming said first implant and said second implant. 
   
   
     16. A method in accordance with  claim 13 , wherein said selecting step comprises determining an as-implanted dopant concentration profile for said first implant. 
   
   
     17. A method in accordance with  claim 16 , wherein said step of determining an as-implanted dopant concentration profile is performed using secondary ion mass spectroscopy. 
   
   
     18. A method in accordance with  claim 13 , further comprising the step of selecting said second peak dopant concentration and said second implant energy such that the base resistance of the varactor device is minimized. 
   
   
     19. A method in accordance with  claim 18 , wherein said selecting step comprises determining an as-implanted dopant concentration profile for said second implant. 
   
   
     20. A method of forming a varactor device on a semiconductor substrate, comprising the steps of:
 forming a buried layer in said semiconductor substrate, said semiconductor substrate having a first conductivity type and said buried layer having a second conductivity type; 
 providing an epitaxial layer situated in said semiconductor substrate, said epitaxial layer having said first conductivity type; 
 forming a first implant in said epitaxial layer using a first implant energy, said first implant having a first peak dopant concentration and said first conductivity type, wherein said first implant extends into said epitaxial layer a first distance, and wherein said first implant is situated over said buried layer; 
 forming a second implant in said epitaxial layer using a second implant energy, said second implant having a second peak dopant concentration and said first conductivity type, wherein said second implant is situated over said buried layer, 
 wherein said second distance is greater than said first distance, wherein said first peak dopant concentration and said first implant energy are selected such that at least one of capacitance, leakage current, and tuning range of the varactor device are optimized, wherein said second peak dopant concentration and said second implant energy are selected with relation to said first peak dopant concentration and said first implant energy such that the base resistance of the varactor device is minimized, 
 and wherein said second peak dopant concentration has a depth that is greater than a depth of said first peak dopant concentration. 
 
   
   
     21. A method in accordance with  claim 20  further comprising the step of forming a contact layer of said first conductivity type overlying said first implant.